Metal-on-metal (MoM) hip implants and pregnancy: are MoM implants good for mom?—a systematic review

Metal-on-metal (MoM) bearing surfaces were historically used for young patients undergoing total hip arthroplasty (THA), and remain commonplace in modern hip resurfacing. A substantial number of female patients with MoM bearings subsequently gave birth following implantation of the bearings before a full understanding of metal ions exposure in these patients was established. In theory, it has been postulated that metal ions released from such implants may cross the placental barrier and cause harm to the fetus. In light of this potential risk, recommendations against the use of MoM components in women of child-bearing age have been advocated. The purpose of this systematic review was to evaluate: (I) the MoM bearing types and ion levels found; (II) the concentrations of metals in maternal circulation and the umbilical cord; and (III) the presence of abnormalities in the fetus or delivered child. A comprehensive literature review was conducted of studies published between January 1st, 1975 and April 1st, 2019 using specific keywords. We defined the inclusion criteria for qualifying studies for this review as follows: (I) studies that reported on the women who experienced pregnancy and who had a MoM hip implant; (II) studies that reported on maternal metal ions blood and umbilical cord levels; and (III) studies that reported on the occurrence of fetal complications. Data on cobalt and chromium ion levels in the maternal blood and umbilical cord blood, as well as the presence of adverse effects in the infant were collected. Age at parturition and time from MoM implant to parturition were also collected. A total of six studies were included in the final analysis that reported on a total of 21 females and 21 infants born. The mean age at parturition was 31 years (range, 24 to 41 years), and the mean time from MoM implantation to parturition was 47 months (range, 11 to 119 months). Maternal blood cobalt levels were found as a weighted average of 34.09 µg/L (0.425 to 138 µg/L), while umbilical cord blood cobalt levels were found to be 22.61 µg/L (0.52 to 51.11 µg/L). Cobalt levels were reduced by an average of 34% between maternal and umbilical cord blood. Maternal cord blood chromium levels were found as a weighted average of 18.18 µg/L (0.225 to 75 µg/L), while umbilical cord chromium levels were found to be 3.96 µg/L (0.14 to 11.96 µg/L). Chromium levels were reduced by an average of 78% between maternal and umbilical cord blood. No cobalt or chromium was detected in the umbilical cord blood of three patients. Out of the 21 infants born to women with MoM implants, 20 were born healthy with no adverse effects or complications. Only one complication was recorded in single infant that did not appear to be related to the maternal MoM implant. To date, there is a lack of consensus as to whether MoM hip arthroplasty implants are to be avoided in the child-bearing female population and whether they constitute a hazard to the fetus in utero. Both chromium and cobalt ions were markedly reduced in levels when transitioning from maternal to cord blood. In particular, chromium showed a greater reduction on average than cobalt (78% vs. 34%). Based on the current evidence, there appears to be no correlation between the presence of metal ions in umbilical cord blood and complications, as none of the infants experienced abnormalities uniquely attributable to the presence of metal ions.